[Streptococcus agalactiae: an unusual cause of septic arthritis beyond 3 months of age. Case report].

Septic arthritis is not a very frequent disease, but with a high morbidity due to the important sequelae that it can cause. The etiology is age-specific, with Staphylococcus aureus being the most frequent microorganism in all ages. Streptococcus agalactiae or group B Streptococcus is an uncommon cause of infection outside the neonatal period. Beyond 3 months of age, infections by this pathogen are associated with serious infections in immunocompromised patients. The treatment of choice is penicillin G or ampicillin. A 4.5-month-old child who developed a group B Streptococcus septic arthritis is reported. The onset was insidious, and the etiological diagnosis prompted us to rule out meningitis and associated immunodeficiency. The extremely low frequency of group B Streptococcus septic arthritis at this age and the importance of ruling out a disseminated disease are crucial learning points in this case.